1. Field of the Invention
The present invention relates generally to automatic chemical cleaning systems, and more specifically to chemical cleaning of electrodes or probes for use in liquor composition measurements in paper pulp processing.
2. Description of the Prior Art
Trees are turned into paper via a series of chemical and mechanical transformations. In the early part of this process, chips of wood are broken down or digested into wood fiber which will subsequently be passed on for mechanical processing, such as drying and pressing. During digestion, the wood chips are loaded into a tank called a digester. A caustic solution--referred to as white liquor--is added to the wood chips. The white liquor breaks down the wood chips into individual wood fibers under heat and pressure. Sodium hydroxide and sodium sulfide, for example, are one commonly used component of white liquor use to break down the wood chips.
Recovery or reclamation of the white liquor for re-use is performed as a secondary process at the paper mill. Reclamation of the caustic solution subsequent to breakdown of the wood chips is typically performed in a two step process called recausticizing in which the used caustic solution is transformed into a black, followed by a green liquor, and finally returned to its white liquor state. The caustic solution may thereafter be reused to separate wood fibers.
For optimum breakdown of the wood fibers in the digester, and for optimum reclamation of the caustic solution in the recausticizing part of the process, accurate control of the composition of the liquor in each step is important. One method of effective composition control is done using a series of electrodes embedded in a housing mounted inside the digester or recausticizing tank. The electrodes measure the chemical composition of the solution in each tank directly, and provide measurement signals to a control system, which then makes any necessary corrections to the liquor composition.
Unfortunately, the electrodes placed in the environment of the digester or recausticizing tank will be prone to buildup of waste material--called scale--during the digesting and recausticizing process steps. Keeping ahead of accumulation of scale on electrodes has long been a difficult challenge in paper mills. As scale builds up on a conventional electrode, the electrode signal drifts, resulting in inaccurate electrode signals and thus inaccurate inputs into process controllers. Historically, this has caused electrodes to be a high-maintenance item for paper mills, and resulted in low confidence in the accuracy of electrode measurements.
In the past, to address these problems the electrodes were typically cleaned or replaced every one to two weeks. During the periods of cleaning or replacement, the liquor composition must be manually controlled, or paper mill processing must be suspended.
As a relatively new alternate solution, ultrasonic cleaning of the electrodes in-situ allows cleaning of the electrode without requiring down-time of the paper mill or manual control or liquor composition. While the ultrasonic system is reliable, it unfortunately cannot keep up with the build-up of scale on the electrodes, and thus the electrodes must be periodically manually cleaned, although at less frequent intervals, than where no in-situ cleaning is performed. Furthermore, the ultrasonic energy applied to the electrodes eventually causes damage to the electrodes, making replacement necessary.